Okay, I have a lot to sah on his matter but I'll keep it short.
Alexandra wasn't in the video because she had trouble getting her VISA while they were shooting for it. Personally I feel like the company could have had her flown out for that few days so that she could be featured in the video for her debut and sent her back home after to handle her issues with her VISA but let's be real they probably don't have the money for that and if they do then they probably find it more of a waste then a profit. They did however film her in America for the teaser, but honestly why put her in the teaser and not the actual video. They definitelt could have given her solo scenes for her raps and filmed in her America for the video as well but I'm gonna get quiet on that.
Moving on. I downloaded the album, haven't listened to it yet besides the title song but I downloaded it. I'm pretty sure Alexandra is only on the title track which leads me to believe she was added to the group after the album was recorded.
I'm assuming, key word assuming, that they added her rap to the title track and decided to have her perform only so they could get a look at how netizens react as well as give her time to learn a bit about the culture in Korea as well as the language before their next comeback which will probably be soon if korean netizens take a liking to Alexandra.